1. Field of the Invention
The present invention relates to a 4-head system rotary head type magnetic recording and reproducing apparatus in which PCM data sent from a computer is recorded.
2. Prior Art
FIG. 8 is a block diagram showing a conventional rotary head type magnetic recording and reproducing apparatus for DAT for recording and reproducing a digital audio signal.
This prior art is of a two-head system, in which a pair of magnetic heads H.sub.1 and H.sub.2 are arranged on a rotary drum 1 at an angle of 180.degree.. The pair of magnetic heads H.sub.1 and H.sub.2 are both used for recording and reproducing. In DAT, an open angle of contact length with respect to the rotary drum 1 of a magnetic tape T is 90 degrees, the tape T running in a direction as indicated at arrow. The rotary drum 1 rotates counterclockwise in the figure, and the pair of magnetic heads H.sub.1 and H.sub.2 alternately scan with respect to the magnetic tape T.
Since the magnetic heads H.sub.1 and H.sub.2 are for recording and reproducing, they are connected to a recording head amplifier 2 and a reproducing head amplifier 3 through a signal transmission means (not shown) such as a rotary transformer. A recording signal processing circuit 4 and a reproducing signal processing circuit 5 are connected to the recording head amplifier 2 and the reproducing head amplifier 3, respectively. A digital audio signal is fed to the recording signal processing circuit 4, and a format including a signal for ATF (Automatic Track Following) or a subcode signal is formed by the recording signal processing circuit 4 and is amplified by the recording head amplifier 2 and recorded on a recording surface of the magnetic tape t by the magnetic heads H.sub.1 and H.sub.2. In the reproducing operation, the magnetic heads H.sub.1 and H.sub.2 scan the magnetic tape T and a signal being recorded is then read and amplified by the reproducing head amplifier 3 and processed by the reproducing signal processing circuit 5. A digital signal output from the reproducing signal processing circuit 5 is demodulated, detected and amplified.
In the conventional rotary type magnetic recording and reproducing apparatus for DAT, the magnetic heads H.sub.1 and H.sub.2 are for recording and reproducing, and therefore, the gap length is also designed for use with the recording and reproducing, as shown in FIG. 8. FIG. 9 shows a magnetic head gap during the recording operation, and FIG. 10 shows a magnetic head gap during the reproducing operation. As shown in these figures, in both the magnetic heads H.sub.1 and H.sub.2, the length T in a direction perpendicularly intersecting with the scanning direction of the gap G is longer than a track pitch (shown at P in FIG. 9). According to a standard of DAT, a track pitch p is 13.6 .mu.m but the length T is in the order of 21 .mu.m. FIGS. 9 and 10 show one magnetic head H.sub.1 but the other magnetic head H.sub.2 has the same gap dimension as that of the magnetic head H.sub.1 and has an azimuth angle reversed to H.sub.1.
The gap length of the magnetic heads H.sub.1 and H.sub.2 is longer than the track pitch p as described above. One reason therefor is that ATF is caused to operate during the reproducing operation. An ATF signal is recorded in areas in both ends of each track recorded on the magnetic tape but during the reproducing, the magnetic heads H.sub.1 and H.sub.2 read ATF signals (pilot signals) of tracks (1 and 2 in FIG. 10) adjacent on both sides of a track (indicated at 2 in FIG. 10) being reproduced, as shown in FIG. 10. By detecting the pilot signals adjacent on both sides as described above, a tracking error direction is detected and a capstan servo is applied accordingly.
The gap length T of the magnetic head is longer than the track pitch p as described above, and therefore, in the recording operation, when data are recorded on the magnetic tape T in order of tracks 1, 2, 3 . . . , an overwrite portion (indicated by hatching in FIG. 9) is formed between the adjacent tracks. There can be obtained the merit in that high density recording free from a clearance between the tracks becomes possible by forming the overwrite portion as described. But there also gives, rise to an inconvenience caused by the overwrite. That is, when an overwrite portion is formed as shown in FIG. 9, a residual magnetism in the overwrite portion cannot be completely removed, and therefore, the S/N ratio of a reproducing signal is lowered by the residual signal in the overwrite portion. FIG. 7 shows a deterioration of signals due to the overwrite. The axis of abscissa indicates the linear recording density, and the axis of coordinates indicates the symbol error rate when an 8-10 modulated signal is reproduced. As shown, as the linear recording density increases, the error rate increases but if an overwrite portion is present, the error rate increases as compared with the case where the overwrite is not present. In the reproducing apparatus for overwrite, demodulation is affected because of the deterioration in signal of the overwrite portion. At the same time, the detection of ATF is adversely affected by the overwrite portion. In the reproducing operation, when an ATF pilot signal of a track to which magnetic head is adjacent is detected, the overwrite portion is also read, and therefore, a residual magnetism results in a turbulence, lowering the tracking accuracy.
In regards to the reproducing operation alone, if the length T of the head gap G increases in order to stabilize the ATF servo, the pilot signal of the adjacent track can be read in a stable manner. However, if the length T unconditionally increases, the width of the overwrite portion shown in FIG. 9 becomes excessively widened during the recording operation. In view of the foregoing, in the conventional apparatus, the gap length is set to be about 1.5 times with respect to the track pitch p. However, the head gap for scanning the adjacent track is thus limited so that, if an undulation occured in the track during recording, the head gap can sometimes miss the ATF area during reproduction.